The present invention relates generally to fiber optic communication devices, and in particular to a tunable optical fiber grating package with low temperature dependency.
Optical gratings are becoming more and more important for selectively controlling specific wavelengths of light transmitted within optical communication systems. Various fiber Bragg gratings (FBG) are a particularly advantageous group of gratings for manipulating optical signals based on their wavelengths. A fiber Bragg grating is generally fabricated by exposing a photosensitive fiber to UV light thereby creating a permanent refractive-index grating along the core of a fiber.
It is familiar to those skilled in the art that the Bragg wavelength xcex of a fiber Bragg grating is related to the period (xcex9) of the fiber Bragg grating and the effective index of refraction (neff) as shown in equation (1):
xcex=2neffxcex9(1)xe2x80x83xe2x80x83(1)
The effective index of refraction neff of the fiber varies significantly over an expected working temperature range of, e.g. xe2x88x9220xc2x0 C. to 80xc2x0 C., mainly due to the thermal expansion of the fiber material-itself and the temperature dependency of the fiber""s effective index of refraction neff. Over this expected working temperature range, the grating wavelength shifting of an uncompensated 1550 nm grating can exceed 1 nm which is not acceptable to an optical communication system.
In equation (1), both the period xcex9 of the fiber Bragg grating and the effective index of refraction neff vary with temperature. The wavelength xcex can be changed by changing the effective index of refraction neff or changing the period xcex9 of the fiber Bragg grating. This principle has been used to develop various devices to compensate wavelength shifting of fiber grating mainly through adjusting the strain applied in the optical fiber.
From equation (1), following equation (2) can be qualitatively derived:
dxcex/xcex=dneff/neff+dxcex9/xcex9xe2x80x83xe2x80x83(2)
According to equation (2), it is apparent to those skilled in the art that the wavelength xcex of a fiber Bragg grating can be changed by changing the effective index of refraction neff and/or by changing the period xcex9. Up to now, the method by changing the effective index of refraction neff is relatively expensive and difficult to achieve. Thus a preferred method is to vary the period xcex9 so as to tune the wavelength of a fiber grating. The period of a fiber Bragg grating is generally varied by stretching, compressing or deforming the fiber. If the effective index of refraction neff is kept constant, the changing of the period of the fiber Bragg grating is proportional to the changing of length of the fiber portion in which the fiber Bragg Grating is written. Unfortunately, the effective index of refraction neff in equation (2) is not constant over an expected temperature range. Therefore after tuning a fiber Bragg grating to an expected wavelength by changing the period, this wavelength is subject to shifting and needs to be compensated against temperature variation.
U.S. Pat. No. 5,042,898 discloses a temperature compensated fiber Bragg device having a fiber portion with two ends. Each end of the fiber portion is attached to a different one of two compensating members made of materials with different positive Coefficients of Thermal Expansion (CTE) relative to one another such that the longitudinal strain applied to the fiber varies with temperature in such a manner that the changes in the central wavelength that are attributable to the changes in the longitudinal strain substantially compensate for those attributable to the changes in the temperature of the grating. This device is relatively complicated and is not suitable for temperature compensation of a tunable device with more than one central wavelength.
U.S. Pat. No. 5,841,920 discloses a similar temperature compensating optical wave-guide device having two supporting components with two different positive Coefficients of Thermal Expansion (CTE). This prior art reference teaches that a single compensating member is not practical since a material possessing the required expansion coefficient properties is not commonly known or readily available (column 1, lines 62-65). This teaching is generally not consistent with the present technical possibilities in the art.
The devices disclosed in U.S. Pat. Nos. 6,144,789 and 6,181,851 also use two supporting components with two different positive Coefficients of Thermal Expansion (CTE). These devices are also not suitable for temperature compensation of a tunable package with more than one central wavelength.
U.S. Pat. No. 6,233,382 teaches a thermal compensating package for an optical fiber Bragg grating having a supporting member which is formed of a composite structure having a first material with a first negative CTE in one direction and a second material with a second CTE in another directions which is different from that of the first CTE. The composite structure is formed of two types (e.g. polymer fibers and carbon fibers) of fibers embedded in an epoxy resin. This device is generally not suitable for temperature compensation of a tunable fiber package. This prior art reference also teaches that it is particularly difficult to provide one single negative CTE material that precisely compensates for temperature variations of a fiber package (column 2, lines 6-9). This teaching is also generally not consistent with the present technical possibilities in the art.
U.S. Pat. No. 6,240,220 teaches a tunable optical fiber grating package which can cause a change in the center wavelength of a fiber Bragg grating. The package provides a controlled and predetermined change in wavelength response by subjecting a fiber portion having a fiber Bragg grating written therein to a controlled strain. The strain in the fiber is induced by varying a longitudinal displacement of a support member which supports the fiber. A PZT actuator is used to linearly displace the fiber support member. Unfortunately, this tunable optical fiber grating package itself has a relatively high temperature dependency due to following reasons:
The support member of the package is made of brass, stainless steel, aluminum, Cu/Be alloy or the like. These materials have relatively high positive CTE and cause the longitudinal displacement of the support member to vary with temperature; and
The supporting member of the package includes a magnification structure with a mechanical advantage between 5 and 15. The variation of the longitudinal displacement of the support member caused by the CTE of the PZT actuator itself is also magnified to an unacceptable amount, e.g. 5 times to 15 times larger than the thermal dimensional variation of the PZT actuator itself.
In view of the above, it would be an advance in the art to provide a tunable optical fiber grating package with low temperature dependency.
It is a primary object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having a first supporting member being formed of a material having ultra low CTE and a second supporting member being formed of a material having a negative CTE. The optical characteristic that varies with strain of the fiber portion, e.g. the wavelength of a Bragg grating, is tunable by changing the dimension of the first supporting member along the longitudinal direction of the fiber by a tuning means. The effect of temperature fluctuation on the fiber grating can be substantially compensated by the second supporting member.
It is another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having an optical fiber portion having at least one grating between a first end and a second end of the fiber portion. The first end and the second end of the fiber portion are fixed on the first supporting member and the second supporting member respectively.
It is yet another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having an optical fiber portion having at least one grating between a first end and a second end of the fiber portion. Both the first end and the second end of the fiber portion are fixed on the second supporting member.
It is yet another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having a first supporting member with an inverse magnification structure (mechanical advantage smaller than 1) that can substantially eliminate the influence of the thermal dimensional variation of the tuning means, e.g. a PZT actuator.
It is yet another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having a first supporting member being formed of a material having ultra low CTE, a third supporting member being formed of a material having ultra low CTE and a second supporting member being formed of a material having a negative CTE. The optical characteristic that varies with strain of the fiber portion, e.g. the wavelength of a Bragg grating, is tuned by changing the dimension of the first, the third supporting members along the longitudinal direction of the fiber by a first and a second tuning means. The effect of temperature fluctuation on the fiber grating can be substantially compensated by the second supporting member.
It is yet another object of the present invention to provide a tunable optical fiber grating package with low temperature dependency having a first supporting member being formed of a material having ultra low CTE, a second supporting member being formed of a material having negative CTE and a third supporting member being formed of a material having a negative CTE. The optical characteristic that varies with strain of the fiber portion, e.g. the wavelength of a Bragg grating, is tuned by changing the dimension of the first supporting members along the longitudinal direction of the fiber by a tuning means. The effect of temperature fluctuation on the fiber grating can be substantially compensated by the second and third supporting members.
By the unique combination of at least one dimension-controllable supporting member being formed of a material having ultra low CTE for tuning and at least one supporting member being formed of a material having a negative CTE for temperature compensation, the tunable optical fiber grating package with low temperature dependency can be used over a predetermined wavelength tuning range for most applications, such as dense wavelength division multiplexing (DWDM) wavelengths latch, reconfigurable optical add/drop multiplexing (OADM), dispersion compensation, tunable fiber laser sources and network channel monitoring and so on. The optical fiber grating package of the present invention is also relatively cost effective and simple in structure.
These and numerous other objects and advantages of the present invention will become apparent upon reading the detailed description.
According to the present invention, there is provided an optical fiber grating package which has a supporting assembly and an optical fiber portion having at least one grating between a first end and a second end of the fiber portion. The first end and the second end of the fiber portion define a longitudinal direction and the at least one grating has an optical characteristic that varies with strain of the fiber portion. The supporting assembly of the optical fiber grating package has a first supporting member being formed of a material having ultra low CTE, a second supporting member being formed of a material having a negative CTE; and a first tuning means for changing the dimension of the first supporting member substantially along the longitudinal direction.
The first supporting member and the second supporting member are bonded together. The first end and the second end of the fiber portion are fixed on the supporting assembly such that the optical characteristic of the grating is tunable by the first tuning means and effect of temperature fluctuations on the strain is substantially compensated by the second supporting member.
The at least one grating includes a plurality of fiber Bragg gratings. Each grating can be a grating selected from a group consisting of even-period grating and uneven-period grating. The optical characteristic includes a characteristic selected from a group consisting of a wavelength of a fiber Bragg grating, a resonant wavelength of Bragg gratings, a change in strain inducing a change in the resonant wavelength of in the fiber portion, and a change in strain within a predetermined range of strain.
In accordance with one aspect of the present invention, the first end of the fiber portion of the optical fiber grating package is fixed on the first supporting member and the second end of the fiber portion is fixed on the second supporting member.
The first supporting member can have an inverse magnification structure with a mechanical advantage smaller than 1 that transfers a displacement of the first tuning means into a change in strain of the fiber portion. In one example of the present invention, the mechanical advantage of the inverse magnification structure is smaller than xc2xd.
The first supporting member can further have a first fiber holder for fixing the first end of the fiber portion, and the second supporting member can have a second fiber holder for fixing the second end of the fiber portion.
The absolute value of the ultra low CTE of the first supporting member is smaller than 2*10xe2x88x926/xc2x0 C. The first supporting member can be made of invar or similar materials with ultra low CTE. The negative CTE of the second supporting member is substantially between xe2x88x927*10xe2x88x926/xc2x0 C. to xe2x88x9214*10xe2x88x926/xc2x0 C. The second supporting member can be made of high negative CTE ceramic composite or similar materials.
The second supporting member supports a predetermined portion of the fiber portion between the first end and the second end of the fiber portion such that the effect of temperature fluctuations on the strain is substantially compensated by the second supporting member.
The first tuning means can have a PZT actuator. The absolute CTE value of the PZT actuator is preferably not larger than 2*10xe2x88x926/xc2x0 C. The first tuning means can also have a servo step motor. The first supporting member can further have a differential screw structure made of the same material as that of the first supporting member. The first supporting member can also have a worm/worm wheel subassembly which can be disposed between a differential screw structure and a servo step motor.
The first supporting member of the optical fiber grating package can also have a magnification structure with a mechanical advantage larger than 1. This is especially preferred when the first tuning means has an ultra-low thermal influence on the dimensional displacement of the first supporting member along the longitudinal direction of the fiber portion. For example, a servo step motor can be used to drive a differential screw structure made of the same ultra low CTE material (e.g. invar) as that of the first supporting member. This configuration can substantially raise the wavelength tuning range.
In accordance with another aspect of the present invention, both the first end and the second end of the fiber portion of the optical fiber grating package are fixed on the second supporting member which is bonded to the first supporting member.
The first supporting member can have an inverse magnification structure with a mechanical advantage smaller than 1 that transfers a displacement of the first tuning means into a change in strain of the fiber portion. In one example of the present invention, the mechanical advantage of the inverse magnification structure is smaller than xc2xd. The second supporting member can further have a first fiber holder for fixing the first end of the fiber portion, and a second fiber holder for fixing the second end of the fiber portion.
The absolute value of the ultra low CTE of the first supporting member is smaller than 2*10xe2x88x926/xc2x0 C. The first supporting member can be made of invar or similar materials with ultra low CTE. The negative CTE of the second supporting member is substantially between xe2x88x927*10xe2x88x926/xc2x0 C. to xe2x88x9214*10xe2x88x926/xc2x0 C. The second supporting member can be made of high negative CTE ceramic composite or similar materials.
The second supporting member supports a predetermined portion of the fiber portion between the first end and the second end of the fiber portion such that the effect of temperature fluctuations on the strain is substantially compensated by the second supporting member.
The first tuning means can have a PZT actuator. The absolute CTE value of the PZT actuator is preferably not larger than 2*10xe2x88x926/xc2x0 C. The first tuning means can also have a servo step motor. The first supporting member can further have a differential screw structure made of the same material as that of the first supporting member. The first supporting member can also have a worm/worm wheel subassembly which can be disposed between a differential screw structure and a servo step motor.
The first supporting member of the optical fiber grating package can also have a magnification structure with a mechanical advantage larger than 1. This is especially preferred when the first tuning means has an ultra-low thermal influence on the dimensional displacement of the first supporting member along the longitudinal direction of the fiber portion. For example, a servo step motor can be used to drive a differential screw structure made of the same ultra low CTE material (e.g. invar) as that of the first supporting member. This configuration can substantially raise the wavelength tuning range.
In accordance with yet another aspect of the present invention, besides the first supporting member being formed of a material having ultra low CTE, the second supporting member being formed of a material having a negative CTE and the first tuning means for changing the dimension of the first supporting member substantially along the longitudinal direction, the supporting assembly of the optical fiber grating package further has a third supporting member being formed of a material having ultra low CTE and a second tuning means for changing the dimension of the third supporting member substantially along the longitudinal direction.
The first supporting member, the third supporting member are bonded with the second supporting member respectively. The first end and the second end of the fiber portion are fixed on the first and third supporting members respectively such that the optical characteristic of the grating is tunable by the first tuning means and the second tuning means, and effect of temperature fluctuations on the strain is substantially compensated by the second supporting member. Each of the first tuning means and the second tuning means can have a tuning means selected from a group consisting of servo step motor and PZT actuator.
In accordance with yet another aspect of the present invention, besides the first supporting member being formed of a material having ultra low CTE, the second supporting member being formed of a material having a negative CTE and the first tuning means for changing the dimension of the first supporting member substantially along the longitudinal direction, the supporting assembly of the optical fiber grating package further has a third supporting member being formed of a material having ultra low CTE and a second tuning means for changing the dimension of the third supporting member substantially along the longitudinal direction.
The first supporting member, the second supporting member and the third supporting member are bonded together along the longitudinal direction with the second supporting member in the middle, the first end and the second end of the fiber portion are fixed on the first and third supporting members respectively such that the optical characteristic of the grating is tunable by the first tuning means and the second tuning means, and effect of temperature fluctuations on the strain is substantially compensated by the second supporting member. Each of the first tuning means and the second tuning means can have a tuning means selected from a group consisting of servo step motor and PZT actuator.
In accordance with yet another aspect of the present invention, besides the first supporting member being formed of a material having ultra low CTE, the second supporting member being formed of a material having a negative CTE and the tuning means for changing the dimension of the first supporting member substantially along the longitudinal direction, the supporting assembly of the optical fiber grating package further has a third supporting member being formed of a material having a negative CTE. The second supporting member and the third supporting member are bonded with the first supporting member respectively.
The first end and the second end of the fiber portion are fixed on the second and third supporting members respectively such that the optical characteristic of the grating is tunable by the tuning means, and effect of temperature fluctuations on the strain is substantially compensated by the second and third supporting members. The second supporting member and the third supporting members can have different negative CTE values.
In accordance with yet another aspect of the present invention, besides the first supporting member being formed of a material having ultra low CTE, the second supporting member being formed of a material having a negative CTE and the tuning means for changing the dimension of the first supporting member substantially along the longitudinal direction, the supporting assembly of the optical fiber grating package further has a third supporting member being formed of a material having a negative CTE.
The first supporting member, the second supporting member and the third supporting member are bonded together along the longitudinal direction with the first supporting member in the middle. The first end and the second end of the fiber portion are fixed on the second and third supporting members respectively such that the optical characteristic of the grating is tunable by the tuning means, and effect of temperature fluctuations on the strain is substantially compensated by the second and third supporting members. The second supporting member and the third supporting member can have different negative CTE values.
The above summary of the present invention is not intended to describe each disclosed embodiment or every implementation of the present invention. The figures and the detailed description will more particularly exemplify these embodiments.